MESOPHYTES. 317 



even in the midst of snow, putting forth their leaves and 

 blossoms while still surrounded by melting snow. 



432. {/) Soil. — Botli the chemical composition and the 

 physical properties of the soil affect plants. The latter are, 

 however, by far the most important. Here, again, the reason 

 is to be found in the relation of the pjhysical cjualities of soil 

 to the water supply. 



The water which permeates the soil takes up from it certain 

 substances, and becomes thus a dilute solution of -^-arious 

 salts. That the salts thus present in the soil water may affect 

 the form of the plant is strikingly shown in the occurrence 

 of certain species of a genus onl)' upjon soils containing lime, 

 while others of the same genus are found only in soils free 

 from lime. When the local distribution of corresponding 

 species of the same genus within the same region is deter- 

 mined by the presence or absence of lime in the soil, com- 

 parison of them indicates the general effect of lime salts upon 

 the plant. Plants growing upon lime are usually stronger 

 and more denseh' hairy, often hoary, while those on other 

 soils are smooth or furnished with glandular hairs. The 

 lime-loving plants ha\'e bluish-green leaves, as contrasted 

 with the gi-ass-green. Their leaves are also more numerous 

 and more deeply branched, the flowers larger and their colors 

 duller and paler. 



